Stratifier and air separator



United States Patent f Superior Separator Company, poration ofDelaware Application March'4, 1953, Serial No. 340,291

3 Claims. (Cl. 209-20) lThis invention relates to improvements in the art of air separation or classification of particles of material and while being particularly useful in conjunction with the classification of mddling stocks in our mills, is also usefulvwherever vfinely comminuted and particulate materials mustbe separated and classified.

Essentially the inventioninvolvesv the separation or classification of materials falling` in a thin curtain into the path of one or more air jets, so` that particles of different terminal velocities l are Vdirected in differing trajectoriesI :into separate collecting receptacles. It is the ,primary object of my invention to provide improvements and refinements in terminal velocity separators Hopkins, Minn., a cor- 2,828,01 l Patented Mar. 25, 1958 ICC Asystat'ed I will at the outset discuss my invention as applied to the size separation and classification of particulate material, and referring to the drawing Vit will be noted that `the material isy supplied tov a hopperV 1 0 having an outlet 11 from which the material falls upon an `elongated feed shoe 12. The feed shoe `is supported for lengthwise vibratory motion upon pivo'ted links 13 and 14 at its opposite ends, which links are in turn p ivoted on any suitable stationary support 15. Whilefpivoted links are here shown, it is obvious that springs might as readily be employed for supporting the shoe and such variation is considered as within the scope of invention. Connected at 16 to the shoe ist the operating eleticles brought to the upper part of thestream,` as clearly of this ,general nature, based upon sound principles and 1 offering a degree of separating and classification eiliciency heretofore impossible to attain. In accordance with my invention 4the particulate material is 'rst of `all delivered overa vibrating feeder shoe, the operation of whichwill vstratify the material and achieve anin- `indicated inthe drawing.` Of course, Vthe shoe also slopes toward its ldeliuery'edge 18 to assist in Vthe feed travel ofthe material..` j j Material fallingfr-,orn .the delivery edge 18 of the shoe, after va free fall-,of-.sorue distance,` is impingedby substantially lhorizontally directed air currents issuing from one. or'more nozzles 1,9., through which air is supplied frorna windbox` IZtihy any suitablemeans, such as' the fan partially indicated at 21. For conveniencev the; v2li-r 1 ustreams issuing from fthe nozzles 19 `will, hereinafter be imparted to the Aparticles Vin -order to separate them accordng to their differing terminal velocities. Thus it will be evident that there are three fundamental consid yerations involved, first of Awhich Ais the initial stratii'cation, secondly the .free fall of the `material before it reaches the jets, so that the length of time in the jet is proportional to the terminal velocity of the falling particles, and finally the `driving of the particlesby a'ilr issuing from the jets into differing trajectories according to their differing terminalvelocities In the interests `of simplicity of this description Vl will first describe -the operation of myV separator 'for size separation Aof particles in the following specification, but 'asstated it rdoes have particular application `to 'the separation and classification of middling` stocks which urequire the separation to be according to diferencestin 'i the shape of 7the particles. When so` used I contemplate still another separation so as to reduce the particles to a nominal size range before they are subjected to the action of the air separator, and it is accordingly another v p object of my invention to 4provide an improved method for the classification of middlingstocks which will permit a highly efficient rseparation of particles which are-of substantially uniform density `and of nominal size range according to differences in theshape of the particles.v

Still *another :object of my invention is to provide an ,referred to as jets and preferably. there are Sever-al -of such nozzles located one 4above the other, .asclejarly v shown. Obviously ythe length of the nozzles, or rather -the width thereof, will be at least equalto the width of `the curtain of material falling from the feed shoe. Below the wind box .20 is another air "chamber 22`into which air is bled from the wind box through a restricted 4orifice -23`formed in `a dividing partition .24, rand-irilwhat may be regarded as the forward'vvall` 251 of the chamber 22 there is a perforated or reticulated area 26 below the nozzles 19. l

Below the material feeding assembly there is a separating chamber 27 and the bottom thereof is provided with a number` `of separate collecting receptacles :28, 2,9, 30 and 31 taking any suitablel form, Such .as iholpers, trays or the like, according to the use' to whichttfhe ap- ,paratus .is to be put.. Preferably, although notnecessarily, the upper edges of these receptacles` `2te-3l fare provided with adjustable baille wings 32 by which-the effective Vopening of the receptacles exposed to fallin material maynbe adjusted 'and-regulated. i

Considered in its vfundamental aspects it is believed that the operation of the apparatusastthus far described is quite apparent and that it will be readlyunderstood that the material falling into the jets issuing from the nozzles 19 will be directed into differing 'trajectories according -to differences in their 'terminal velocities vand thus will be separately collected in the receptacles 28-31. lt is to be noted, and it is very important, that`v there .is a

' 'counterflow ofthe material beingseparated, as indicated by the arrow A, 'and of the air jets issuing .from the nozzles 19, as indicated by the arrow B. Since we arefherecon* sidering the separation and classification-of .material :accordingwto size the larger particles of material are indif -catedl'at .iP `and the particles of diminishing size are indicatedat SP1, P2 and P3. The' counterow tof the material :and -air ,is important for the reason that, due to the stratification-:cf the material by the` feed shoe the smaller particles i-n the falling `curtain of `material are in effect ffbehind vor downstream from thclarger particles,

ty and size.

or further from the nozzles 19, and it istherefore unnecessary to blow the smaller particles through the larger during separation, as would be the case were there no counterliow and no initial stratificationV of the material.

Turning now to the more critical aspects of the invention which are so important to the eflicientair separation and classification of materials, it may be said that there are three fundamentals involved, or in reality three distinct, consecutive steps in the separation. The first of these is the stratification of the material by the vibratory feeder, as outlined Supra, andrin addition to this action bringing thecoarser particles to the top of the stream of the material on the shoe it will also be noted that these coarser particles will fall in a trajectory further away from the delivery edge 18 than will the finer particles. This, then, is the initial stage of the separation of the coarse from the fine particles.

Thel free fallof this stratified material before it reaches the ail jets causes the length of time in the jet to be proportional to the terminal velocity of the falling particles and the coarser particles may fall faster than the fines, or put in another way, the coarser particlesV may have a greater relative downward velocity. The impulse Vtransmitted to a particle falling through the air jet is governed by the formula: Y

Impulse=rair drag on particleXtime in jet Thus it will be apparent that the relative horizontal deflection ofthe particles by the air jets will be proportional to the impulse given to the particles and thus, since l allow the particles to fall a short distance before they reach the jets, they are in the jet a time proportional to their terminal velocities in air and this is the second step or stage of separation.

The above equation shows that the impulse given by the jetis proportional to the air drag on the particle multiplied byrthe time in the jet and there are two expressions which relate the terminal velocity of the particle to its size, shape, etc. The first of these expressions or formulae is known as Stokes law and applies if the flow of iiuid (which in this case of course is air) around the particle is streamline. The second applies ifV the flow of the air around the particle is turbulent and theselexpressions are as follows:

where B Turbulent ow V51=K2 V; d

where again VT is the terminal velocity p the density difference d the particle size, but

K2 is a constant dependent on the particle shape in turbulent flow.

Since the density and viscosity of the air may be regarded as constant for'practical purposes, the terminal velocity of the particles thus depends onl the shape, densi- Experimentally it has been found that the first expres'- sion A given above applies up to a terminal velocity of about eighty feet per minute and that the second applies above a terminal Velocity in theeneighborhood of four hundred feet per minute. fiow from eighty to four hundred feet per minute is a transition region. Y It is' important to choose the velocity according to the nature of the separation being made The intervening region 'of' and where the separation is by size, conditions applying in the expression A are most desirable, but considering the application of the separator to the application of middling stocks, which may be made up of rather squarish piecesof endosperm, commingled with flat pieces of bran, the differences between the density of these particles can be regarded as insignificant thus leaving only the shape and size of the particles of importance in determining their terminal velocities in the air jet. A comparison of the above expressions shows that B is the least dependent upon size and density but the most dependent upon shape, and thus conditions suitable to separation under the second expression, or above four hundred feet per minute are most desirable for separation by shape. Thus jet velocities high enough to bring the relative velocity between jet and particle into the turbulent region are used Yand in my experiments I have used velocities in the fifteen hundred to twenty-five hundred range.

Itis quite apparent that a small, squarish particle of endosperm might have the same treminal velocity as a larger, iiat piece ofrbran and Vthus effective separation of middling stocks can only be satisfactorily accomplished in a terminal velocity separator if there is a preliminary 'sifting or sizingroperation to reduce the stocks to approximately a nominalparticle size.

I have accordingly in the drawing indicated in a block diagram S is sifting or sizing unit, for reducing the material to a nominal size range, as supplying the material to the hopper 10 and regard as one aspect of lmy invention the method of terminal velocity separation of stocks according to differences in particle shape which includes as a first step the sizing of the particle and subsequently the stratification and free fall of the material by the feeder and finally the separation by a counteriiow of the particle and thev air used for imparting velocity to the particles.

It will be apparent that the particle size range is important and I have determined by extensive experimentation that the efficiency of separation is sufficient for commercial purposes where the range in particle size is within, say, thirty percent ofthe mean particle size. This can be statedas the difference between the mesh opening inthe screen through Vwhich the particles pass (throughs) and the mesh opening in the screen on which the particles are retained `(overs) should'not exceed thirty percent of Ythe meanof the two VVsizes of mesh. Reduced Vto formula thisV is as'4 follows:

Through mesh dmensionover mesh dirnension Mean of through and over mesh dimension Y Y Y less than 30% Careful consideration offboth sifting and separation and Vproper'application of the principles thereof make it posfollows: .n

' Table 1 Y f il Percent of Air Separation A Total Ash i Stream It will he obvious to those skilled inithis field that, following air separation, the particles recovered in the Table 2 Percent ol Air Separation Total Ash Stream Such analysis will demonstrate that there remains some bran in the stock which could be removed but separation 1 and 2 are very clean and may be reduced immediately to our. Separation 3 indicates that suicient bran remains and gradual reduction is necessary while separation 4 will obviously be branny and quite grey in color and should be sent on to the tail of the mill.

While I have herein discussed my invention in detail in its adaptation to flour mill uses, it is to be understood that I do not limit myself to this particular field. Obviously the separator will have many other uses, such as for the separation and classification of metal powders and other comminuted products which are diicult if not impossible to separate with any worthwhile efficiency by prior apparatus.

Referring again to the drawing, the provision of the chamber 22 and perforated area 26 to which air is fed from the wind box 20 is designed to provide a very slight plus pressure in the zone beneath the nozzles 19 without which there is a tendency for the lighter or finer particles to collect at this point. The air streams issuing from the nozzles are believed to create a minus pressure zone beneath them and it is believed sufficient if the air supplied at 26 just overcomes this tendency toward the creation of such minus pressure.

One great advantage of my invention lies in its extreme simplicity and its very low power requirements as compared to other separators of present forms. As an example, and again referring to flour mill usage, a separator capable of handling 300 pounds per hour of middlings would require only 150 watts to power both the vibrator and to supply air to the nozzles. Space requirements, which are always a problem, are also nominal for the separator of my invention.

It will, of course, be understood that the shoe 12 will have sufficient width to stratify the material in a comparatively thin stream and that the nozzles 19 will also have suicient width to act upon the full width of the stream or curtain of material falling from the shoe. The vertically spaced series of nozzles is desirable so that the air will act on the stream as it separates into its component particle P, P1, etc., to insure a proper Separation.

It is understood that suitable modifications may be made in the structure as disclosed, provided such modifications come within the spirit and scope of the appended 6 claims. Having now therefore fully illustrated and described my invention, what I claim to be new and desire to protect by Letters Patent is:

1. Apparatus for air separation of particulate material, comprising in combination, a feed shoe having a discharge end, means for supplying the material to be separated to the shoe, means for vibrating the shoe to feed the material along the shoe in one direction and to stratify the material according to differences in its particles as the material falls from the discharge end of the shoe, a series of vertically spaced air jet nozzles located to direct air jets into the material falling from the shoe and to direct said air in a direction opposite that traveled by the material over the shoe whereby there is a counterflow of the material and of the air, means for supplying air to the nozzles, separate receptacles located below the nozzles for collecting the particles of differing terminal velocities given differing trajectories by the air from the nozzles, and means for discharging air into a zone below the nozzles generally horizontally and in an amount sufficient to prevent the air jets issuing from those nozzles from creating a minus pressure at said zone.

2. An air jet separator of the character described, comprising means for stratifying a particulate material in a generally horizontally moving stream and then discharging the material in a falling curtain, air jet means located adjacent the path of the material and directing air across the full width of the curtain thereof and in a substantially horizontal zone substantially below the discharge of said material after the same has had a substantial free fall and in a direction such that the air will first impinge the stratum of material brought to the top of the stream before it falls, means for supplying the air under pressure to said jet means, means for separately collecting` the particles of the material of differing terminal velocities given different trajectories by the air under pressure, and means for also supplying air in a zone below said air jet means to counteract the tendency of the air from said jets to create a negative pressure in said zone.

3. Apparatus for air separation of particulate material, comprising in combination, a feed shoe having a discharge end, means for supplying the material to be separated to the shoe, means for vibrating the shoe to feed the material along the shoe in one direction and to stratify the material according to differences in its particles as the material falls from the discharge end of the shoe, a series of vertically spaced air jet nozzles located to direct air jets into the material falling from the shoe and to direct said air in a direction opposite that traveled by the material over the shoe whereby there is a counterow of the material and of the air, a wind box supplying air to said nozzles, a series of receptacles below the nozzles and spaced in a horizontal direction therefrom for separately collecting the particles separated by the air from the falling material, means forming an air chamber below the nozzles and having an upright perforated wall, and means for bleeding air into the said air chamber for discharge through said wall outward below the nozzles.

References Cited in the file of this patent UNITED STATES PATENTS 293,103 Scottiaux Feb. 5, 1884 FOREIGN PATENTS 279,041 Italy Oct. 29, 1930 424,698 Germany Ian. 29, 1926 OTHER REFERENCES Rock Products, volume 54, Number 9, September 1951, pages to 77.

Taggart: Handbook of Mineral Dressing, 1945, section 11, page 03. 

